Magnetic disk drives including a hard disk drive, have data recorded in circumferential or spiral loci (tracks) formed on a disk such as a magnetic recording medium. For writing or reading of the data, control (positioning control) is extended to move a magnetic head to any of the tracks.
In conventional positioning control, a signal is processed via a filter that degrades sensitivity at a frequency corresponding to the frequency of vibrations inherent to a housing. However, this causes sensitivity functions relevant to other frequencies to rise, or, anyhow, adversely affect the sensitivity functions relevant to the other frequencies. Moreover, since the transfer function of a control circuit assumes a real number value at a Nyquist frequency, a phase is always 0° or 180°. As a frequency gets closer to the Nyquist frequency, it is hard to improve the sensitivity function by advancing the phase.
U.S. Patent Publication No. 2003/161065 (“Patent Document 1”) or the like, has disclosed a technology for expanding a controllable frequency range by adopting a multi-rate estimator for a feedback control system.